Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized as either continuous ink jet or drop-on-demand ink jet. U.S. Pat. No. 3,946,398, which issued to Kyser et al. in 1970, discloses a drop-on-demand ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Other types of piezoelectric drop-on-demand printers utilize piezoelectric crystals in push mode, shear mode, and squeeze mode. Piezoelectric drop-on-demand printers have achieved commercial success at image resolutions up to 720 dpi for home and office printers. However, piezoelectric printing mechanisms usually require complex high voltage drive circuitry and bulky piezoelectric crystal arrays, which are disadvantageous in regard to manufacturability and performance.
Great Britain Patent No. 2,007,162, which issued to Endo et al. in 1979, discloses an electrothermal drop-on-demand ink jet printer which applies a power pulse to an electrothermal heater which is in thermal contact with water based ink in a nozzle. A small quantity of ink rapidly evaporates, forming a bubble which cause drops of ink to be ejected from small apertures along the edge of the heater substrate. This technology is known as Bubblejet.TM. (trademark of Canon K.K. of Japan).
U.S. Pat. No. 4,490,728, which issued to Vaught et al. in 1982, discloses an electrothermal drop ejection system which also operates by bubble formation to eject drops in a direction normal to the plane of the heater substrate. As used herein, the term "thermal ink jet" is used to refer to both this system and system commonly known as Bubblejet.TM..
Thermal ink jet printing typically requires a heater energy of approximately 20 .mu.J over a period of approximately 2 .mu.sec to heat the ink to a temperature between 280.degree. C. and 400.degree. C. to cause rapid, homogeneous formation of a bubble. The rapid bubble formation provides the momentum for drop ejection. The collapse of the bubble causes a tremendous pressure pulse on the thin film heater materials due to the implosion of the bubble. The high temperatures needed necessitates the use of special inks, complicates the driver electronics, and precipitates deterioration of heater elements. The 10 Watt active power consumption of each heater is one of many factors preventing the manufacture of low cost high speed pagewidth printheads.
U.S. Pat. No. 4,275,290, which issued to Cielo et al., discloses a liquid ink printing system in which ink is supplied to a reservoir at a predetermined pressure and retained in orifices by surface tension until the surface tension is reduced by heat from an electrically energized resistive heater, which causes ink to issue from the orifice and to thereby contact a paper receiver. This system requires that the ink be designed so as to exhibit a change, preferably large, in surface tension with temperature. The paper receiver must also be in close proximity to the orifice in order to separate the drop from the orifice.
U.S. Pat. No. 4,166,277, which also issued to Cielo et al., discloses a related liquid ink printing system in which ink is supplied to a reservoir at a predetermined pressure and retained in orifices by surface tension. The surface tension is overcome by the electrostatic force produced by a voltage applied to one or more electrodes which lie in an array above the ink orifices, causing ink to be ejected from selected orifices and to contact a paper receiver. The extent of ejection is claimed to be very small in the above Cielo patents, as opposed to an "ink jet", contact with the paper being the primary means of printing an ink drop. This system is disadvantageous, in that a plurality of high voltages must be controlled and communicated to the electrode array. Also, the electric fields between neighboring electrodes interfere with one another. Further, the fields required are larger than desired to prevent arcing, and the variable characteristics of the paper receiver such as thickness or dampness can cause the applied field to vary.
In U.S. Pat. No. 4,751,531, which issued to Saito, a heater is located below the meniscus of ink contained between two opposing walls. The heater causes, in conjunction with an electrostatic field applied by an electrode located near the heater, the ejection of an ink drop. There are a plurality of heater/electrode pairs, but there is no orifice array. The force on the ink causing drop ejection is produced by the electric field, but this force is alone insufficient to cause drop ejection. That is, the heat from the heater is also required to reduce either the viscous drag and/or the surface tension of the ink in the vicinity of the heater before the electric field force is sufficient to cause drop ejection. The use of an electrostatic force alone requires high voltages. This system is thus disadvantageous in that a plurality of high voltages must be controlled and communicated to the electrode array. Also the lack of an orifice array reduces the density and controllability of ejected drops.
There has been proposed a liquid printing system that affords significant improvements toward overcoming the prior art problems associated with drop size and placement accuracy, attainable printing speeds, power usage, durability, thermal stresses, other printer performance characteristics, manufacturability, and characteristics of useful inks. There is provided a drop-on-demand printing mechanism wherein the means of selecting drops to be printed produces a difference in position between selected drops and drops which are not selected, but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body of ink, and wherein an additional means is provided to cause separation of said selected drops from said body of ink. The following table entitled "Drop separation means" shows some of the possible methods for separating selected drops from the body of ink, and ensuring that the selected drops form dots on the printing medium. The drop separation means discriminates between selected drops and un-selected drops to ensure that unselected drops do not form dots on the printing medium.
__________________________________________________________________________ Drop separation means Means Advantage Limitation __________________________________________________________________________ Electrostatic Can print on rough surfaces, Requires high voltage power attraction simple implementation supply AC electric field Higher field strength is Requires high voltage AC possible than electrostatic, power supply synchronized to operating margins can be drop ejection phase. Multiple increased, ink pressure drop phase operation is reduced, and dust difficult accumulation is reduced Proximity (printhead Very small spot sizes can be Requires print medium to be in close proximity to, achieved. Very low power very close to printhead surface, but not touching, dissipation. High drop unsuitable for rough print recording medium) position accuracy media, usually requires transfer roller or belt Transfer Proximity Very small spot sizes can be Not compact due to size of (printhead is in close achieved, very low power transfer roller or transfer belt proximity to a dissipation, high accuracy, transfer roller or belt can print on rough paper Proximity with Useful for hot melt inks using Requires print medium to be oscillating ink viscosity reduction drop very close to printhead surface, pressure selection method, reduces not suitable for rough print possibility of nozzle clogging, media. Requires ink pressure can use pigments instead of oscillation apparatus dyes Magnetic attraction Can print on rough surfaces. Requires uniform high Low power if permanent magnetic field strength, magnets are used requires magnetic ink __________________________________________________________________________
The proposed liquid printing system affords significant improvements toward overcoming problems associated with drop size and placement accuracy, attainable printing speeds, power usage, durability, thermal stresses, other printer performance characteristics, manufacturability, and characteristics of useful inks.
An ink jet printer can comprise several systems: the printheads that can utilize one of the above described printing method, an ink delivery system that supplies the ink to the printhead, a printhead transport system that transports the printhead across the page, a receiver transport system that moves receiver medium across the printhead for printing, an image data process and transfer system that provides digital signal to the printhead, a printhead service station that cleans the printhead, and the mechanical encasement and frame that support all above systems.
The ink delivery system in an ink jet printer may exist in several forms. In most page-size ink jet printers, the ink usage is relatively low. The ink is stored in a small cartridge that is attached to, or built in one unit with, the printhead. Examples of the ink cartridges are disclosed in U.S. Pat. Nos. 5,541,632 and 5,557,310. In large format inkjet printers, the ink usage per print is usually high. Auxiliary ink reservoirs are required to store large volumes of ink fluid that are connected to the ink cartridges near the printheads. Examples of auxiliary ink reservoirs are disclosed in European Patents EP 0 745 481 A2 and EP 0 745 482 A2. The level of the ink residual quantity can also be detected. For example, U.S. Pat. No. 5,250,957 discloses an ink detector that senses ink by measuring the electric resistance in the ink.
One problem for ink jet printing is in the variabilities in the physical properties and the chemical compositions in the ink. These variabilities can be caused by ink aging, or mismatching the wrong types of inks to a printer and receiver medium. The variabilities in the ink physical properties and ink chemical compositions compromise the ideal performance of the ink jet printers. For example, print density and color balance can be adversely affected by variations in the physical properties of the ink. These adverse effects can occur within a print, between prints of a given printer, and/or between prints from different printers. Print failures such as in-jet nozzle plugging can also occur as a result of the above described variabilities.